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Old 01-29-2010, 09:43 AM
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anderkart anderkart is offline
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Originally Posted by frederic View Post
You can drive automotive alternators with a pull-start engine. You still need the battery to provide the field voltage as the alternator is "excited" while it speeds up. Once at a certain, minimum speed the alternator then charges the battery as it does in a car, truck or boat. If you remove the battery while the alternator is providing current, you can easily overheat and damage the alternator windings as well as the internal current regulator. The battery, and it's resistance, is integral to the current regulation circuit.

Automotive engines typically do about 2500-2700 rpms for highway speed, so alternators are pulleyed to about a 3:1 or 4:1 ratio. For a gasoline engine that produces it's peak horsepower close or at it's redline of 3500 RPM, you could gear things a little closer. For durability you'd want to increase the pulley diameters but keep the ratio the same, so that more belt surface touches the pulley. This will compensate a bit for the conditions that homemade systems of this kind must endure - like sitting in the freezing garage for six months allowing the belt to get a "memory" of it's shape.

Also know that temperature of the alternator windings and casing impacts the amount of current you can draw. The colder it is, the more you can draw. This is why automotive alternators have an integrated fan, and hope that the car is moving more than it is idling. I'll give you a tip - large case alternators generally can provide more current than small frame alternators earlier in the RPM range for a given temperature. If you can get a large frame alternator that's generally better however whatever you get at a junkyard, you can drive with a lawnmower engine no problem.
+1, excellent info frederic. (and nerfo)

I spent serveral years rebuilding/bench testing Alternators. We had 2 test-benches, one had a 3hp and the other a larger 5hp electric motor to spin up the Alternators. I found the lower powered test bench did not have the necessary power to spin Alternators over around 70-amps up to full output. While our 5-hp bench tester could easly spin up Alternators with max outputs of up to 150+ amps.

While I agree that the type of large case/frame Alternators (like 70's/80's Police cars came with) usualy produced higher output at lower RPM's, I just wanted to point out that any Alternator with over about a 30 amp max output would most likely end up being overkill for most any go kart and take way too much of the small engines avalible power away from driving the kart.

A go karts limited electrical demands wouldnt necessarily cause any Alternator to charge at full tilt all the time, but a partly discharged battery could for awhile...

If I were to choose an automotive Alternator to instal on a go kart, I'd probably go with a Delco 10SI (Lester part #7127) rebuilt to a lower max-output level. This Alternator model is inexpensive, reliable, easy to repair and have a built in regulator. There are Self-energising regulators avalible for these and when used the alternator simply needs 1 (10-gauge) wire ran to the battery's positive post to function. Most of these 10SI alternators are commonly rebuilt to around a 65 amp (or so) max-output level, but if you deal directly with a rebuild shop they can usualy equip it with a lighter duty Stator (and even Rotor) to lower its max output level closer to 30-amps and leave more of the engines avalible power to actualy drive the kart.